Systems and methods for protecting power conversion systems based on at least feedback signals

ABSTRACT

System and method for protecting a power conversion system. An example system controller includes a protection component and a driving component. The protection component is configured to receive a demagnetization signal generated based on at least information associated with a feedback signal of the power conversion system, process information associated with the demagnetization signal and a detected voltage generated based on at least information associated with the feedback signal, and generate a protection signal based on at least information associated with the detected voltage and the demagnetization signal. The driving component is configured to receive the protection signal and output a driving signal to a switch configured to affect a primary current flowing through a primary winding of the power conversion system. The detected voltage is related to an output voltage of the power conversion system. The demagnetization signal is related to a demagnetization period of the power conversion system.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201310078547.9, filed Mar. 12, 2013, commonly assigned, incorporated byreference herein for all purposes. In addition, this application is acontinuation-in-part of U.S. patent application Ser. No. 13/071,384,filed Mar. 24, 2011, claiming priority to Chinese Patent Application No.201110051423.2, filed Feb. 28, 2011, both of these applications commonlyassigned and incorporated by reference herein for all purposes.Moreover, U.S. patent application Ser. No. 13/071,384 is acontinuation-in-part of U.S. patent application Ser. No. 12/581,775,filed Oct. 19, 2009, claiming priority to U.S. Provisional No.61/107,249, filed Oct. 21, 2008, both of these applications commonlyassigned and incorporated by reference herein for all purposes.

Additionally, this application is related to U.S. patent applicationSer. No. 12/502,866, commonly assigned, incorporated by reference hereinfor all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for protectinga power conversion system based on at least a feedback signal. Merely byway of example, the invention has been applied to a flyback powerconversion system. But it would be recognized that the invention has amuch broader range of applicability.

Generally, a conventional power conversion system often uses atransformer to isolate the input voltage on the primary side and theoutput voltage on the secondary side. To regulate the output voltage,certain components, such as TL431 and an opto-coupler, can be used totransmit a feedback signal from the secondary side to a controller chipon the primary side. Alternatively, the output voltage on the secondaryside can be imaged to the primary side, so the output voltage iscontrolled by directly adjusting some parameters on the primary side.Then, some components, such as TL431 and an opto-coupler, can be omittedto reduce the system costs.

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system with primary-side sensing and regulation. The powerconversion system 100 includes a primary winding 110, a secondarywinding 112, an auxiliary winding 114, a power switch 120, a currentsensing resistor 130, an equivalent resistor 140 for an output cable,resistors 150 and 152, and a rectifying diode 160. For example, thepower switch 120 is a bipolar junction transistor. In another example,the power switch 120 is a MOS transistor.

To regulate the output voltage within a predetermined range, informationrelated to the output voltage and the output loading often needs to beextracted. For example, when the power conversion system 100 operates ina discontinuous conduction mode (DCM), such information can be extractedthrough the auxiliary winding 114. When the power switch 120 is turnedon, the energy is stored in the secondary winding 112. Then, when thepower switch 120 is turned off, the stored energy is released to theoutput terminal during a demagnetization process. The voltage of theauxiliary winding 114 maps the output voltage on the secondary side asshown below.

$\begin{matrix}\begin{matrix}{V_{FB} = {\frac{R_{2}}{R_{1} + R_{2}} \times V_{aux}}} \\{= {k \times n \times \left( {V_{o} + V_{F} + {I_{o} \times R_{eq}}} \right)}}\end{matrix} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where V_(FB) represents a voltage at a node 154, and V_(aux) representsthe voltage of the auxiliary winding 114. R₁ and R₂ represent theresistance values of the resistors 150 and 152 respectively.Additionally, n represents a turns ratio between the auxiliary winding114 and the secondary winding 112. Specifically, n is equal to thenumber of turns of the auxiliary winding 114 divided by the number ofturns of the secondary winding 112. V_(o) and I_(o) represent the outputvoltage and the output current respectively. Moreover, V_(F) representsthe forward voltage of the rectifying diode 160, and R_(eq) representsthe resistance value of the equivalent resistor 140. Also, k representsa feedback coefficient as shown below:

$\begin{matrix}{k = \frac{R_{2}}{R_{1} + R_{2}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

FIG. 2 is a simplified diagram showing a conventional operationmechanism for the flyback power conversion system 100. As shown in FIG.2, the controller chip of the conversion system 100 uses asample-and-hold mechanism. When the demagnetization process on thesecondary side is almost completed and the current I_(sec) of thesecondary winding 112 almost becomes zero, the voltage V_(aux) of theauxiliary winding 114 is sampled at, for example, point A of FIG. 2. Thesampled voltage value is usually held until the next voltage sampling isperformed. Through a negative feedback loop, the sampled voltage valuecan become equal to a reference voltage V_(ref). Therefore,V _(FB) =V _(ref)  (Equation 3)

Combining Equations 1 and 3, the following can be obtained:

$\begin{matrix}{V_{o} = {\frac{V_{ref}}{k \times n} - V_{F} - {I_{o} \times R_{eq}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$Based on Equation 4, the output voltage decreases with the increasingoutput current.

Additionally, in the discontinuous conduction mode (DCM), the flybackpower conversion system 100 can also regulate the output currentregardless of the output voltage based on information associated withthe waveform for the voltage V_(aux) of the auxiliary winding 114 asshown in FIG. 2.

For example, the output current is equal to an average of a secondarycurrent 198 flowing through the secondary winding 112 during a switchingperiod which includes a demagnetization period corresponding to thedemagnetization process.

$\begin{matrix}{I_{old} = {\frac{1}{2}I_{sec\_ pk}\frac{T_{dem}}{T_{s}}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$where I_(out) represents the output current, I_(sec) _(_) _(pk)represents the magnitude of the secondary current 198 when the switch120 is turned off, T_(dem) represents the duration of thedemagnetization period, and T_(s) represents the duration of theswitching period.

As an example, according to Equation 5, the output current can bedetermined as follows:

$\begin{matrix}{I_{out} = {\frac{1}{2}N\frac{1}{T}{\int_{0}^{T}{\frac{V_{cs}}{R_{s}}\frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$where N represents the turns ratio between the primary winding 110 andthe secondary winding 112, R_(s) represents the resistance of theresistor 130, T represents an integration period, and V_(cs) representsa peak current-sensing signal associated with a primary current 196flowing through the primary winding 110 in each switching cycle.

According to Equation 6, if V_(cs) and T_(dem)/T_(s) do not change much,the output current may be regulated regardless of the input voltage, theoutput voltage, or the inductance of the transformer including theprimary winding 110 and the secondary winding 112, so the powerconversion system 100 operates, for example, in a constant-current mode.

But when the power conversion system 100 operates in theconstant-current mode, the power conversion system 100 needs to beprotected. Hence it is highly desirable to improve the techniques ofsystem protection.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for protectinga power conversion system based on at least a feedback signal. Merely byway of example, the invention has been applied to a flyback powerconversion system. But it would be recognized that the invention has amuch broader range of applicability.

According to one embodiment, a system controller for protecting a powerconversion system includes a protection component and a drivingcomponent. The protection component is configured to receive ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system,process information associated with the demagnetization signal and adetected voltage generated based on at least information associated withthe feedback signal, and generate a protection signal based on at leastinformation associated with the detected voltage and the demagnetizationsignal. The driving component is configured to receive the protectionsignal and output a driving signal to a switch configured to affect aprimary current flowing through a primary winding of the powerconversion system. The detected voltage is related to an output voltageof the power conversion system. The demagnetization signal is related toa demagnetization period of the power conversion system. The protectioncomponent and the driving component are further configured to, if thedetected voltage and the demagnetization signal satisfy one or moreconditions, output the driving signal to cause the switch to open andremain open in order to protect the power conversion system.

According to another embodiment, a system controller for protecting apower conversion system includes a protection component and a drivingcomponent. The protection component is configured to receive ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system,receive a current-sensing signal associated with a primary currentflowing through a primary winding of the power conversion system,process information associated with the demagnetization signal, thecurrent-sensing signal, and a detected voltage generated based on atleast information associated with the feedback signal, and generate aprotection signal based on at least information associated with thedetected voltage, the demagnetization signal, and the current-sensingsignal. The driving component is configured to receive the protectionsignal and output a driving signal to a switch configured to affect theprimary current flowing through the primary winding. The detectedvoltage is related to an output voltage of the power conversion system.The demagnetization signal is related to a demagnetization period of thepower conversion system. The protection component and the drivingcomponent are further configured to, if the detected voltage, thedemagnetization signal and the current-sensing signal satisfy one ormore conditions, output the driving signal to cause the switch to openand remain open in order to protect the power conversion system.

In one embodiment, a method for protecting a power conversion systemincludes, receiving a demagnetization signal generated based on at leastinformation associated with a feedback signal of the power conversionsystem, processing information associated with the demagnetizationsignal and a detected voltage generated based on at least informationassociated with the feedback signal, and generating a protection signalbased on at least information associated with the detected voltage andthe demagnetization signal. The method further includes, receiving theprotection signal, generating a driving signal based on at leastinformation associated with the protection signal, and outputting thedriving signal to a switch configured to affect a primary currentflowing through a primary winding of the power conversion system. Thedetected voltage is related to an output voltage of the power conversionsystem. The demagnetization signal is related to a demagnetizationperiod of the power conversion system. The process for outputting adriving signal to a switch configured to affect a primary currentflowing through a primary winding of the power conversion systemincludes, if the detected voltage and the demagnetization signal satisfyone or more conditions, outputting the driving signal to cause theswitch to open and remain open in order to protect the power conversionsystem.

In another embodiment, a method for protecting a power conversion systemincludes, receiving a demagnetization signal generated based on at leastinformation associated with a feedback signal of the power conversionsystem, receiving a current-sensing signal associated with a primarycurrent flowing through a primary winding of the power conversionsystem, and processing information associated with the demagnetizationsignal, the current-sensing signal, and a detected voltage generatedbased on at least information associated with the feedback signal. Themethod further includes, generating a protection signal based on atleast information associated with the detected voltage, thedemagnetization signal, and the current-sensing signal, receiving theprotection signal, generating a driving signal based on at leastinformation associated with the protection signal, and outputting thedriving signal to a switch configured to affect the primary currentflowing through the primary winding. The detected voltage is related toan output voltage of the power conversion system. The demagnetizationsignal is related to a demagnetization period of the power conversionsystem. The process for outputting a driving signal to a switchconfigured to affect the primary current flowing through the primarywinding includes, if the detected voltage, the demagnetization signaland the current-sensing signal satisfy one or more conditions,outputting the driving signal to cause the switch to open and remainopen in order to protect the power conversion system.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system with primary-side sensing and regulation.

FIG. 2 is a simplified diagram showing a conventional operationmechanism for the flyback power conversion system as shown in FIG. 1.

FIG. 3 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation.

FIG. 4 is a simplified diagram showing at least certain components ofthe constant-current component as part of the power conversion system asshown in FIG. 3.

FIG. 5 is a simplified diagram showing at least certain components ofthe demagnetization detector as part of the power conversion system asshown in FIG. 3.

FIG. 6 is a simplified timing diagram for the power conversion system asshown in FIG. 3.

FIG. 7(a) is a simplified diagram showing a relationship between theoperating frequency and the output voltage of the power conversionsystem as shown in FIG. 3 in the constant-current mode under normaloperations.

FIG. 7(b) is a simplified diagram showing a relationship between theduration of the demagnetization period and the output voltage of thepower conversion system as shown in FIG. 3 in the constant-current modeunder normal operations.

FIG. 8 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation according to an embodiment of thepresent invention.

FIG. 9 is a simplified diagram showing certain components of theprotection component as part of the power conversion system as shown inFIG. 8 according to an embodiment of the present invention.

FIG. 10 is a simplified diagram showing a relationship between theduration of the demagnetization period and the signal of the powerconversion system as shown in FIG. 8 under normal operations, and undercertain abnormal operations against which the power conversion system isprotected, according to one embodiment of the present invention.

FIG. 11 is a simplified diagram showing certain components of theprotection component as part of the power conversion system as shown inFIG. 8 according to another embodiment of the present invention.

FIG. 12 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation according to another embodiment ofthe present invention.

FIG. 13 is a simplified diagram showing certain components of theprotection component as part of the power conversion system as shown inFIG. 12 according to one embodiment of the present invention.

FIG. 14 is a simplified diagram showing a relationship between theduration of the demagnetization period and the signal of the powerconversion system as shown in FIG. 12 under normal operations, and undercertain abnormal operations against which the power conversion system isprotected, according to an embodiment of the present invention.

FIG. 15 is a simplified diagram showing certain components of theprotection component as part of the power conversion system as shown inFIG. 12 according to another embodiment of the present invention.

FIG. 16 is a simplified diagram showing certain protection processimplemented by the power conversion system as shown in FIG. 8 and/or thepower conversion system as shown in FIG. 12 according to certainembodiments of the present invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for protectinga power conversion system based on at least a feedback signal. Merely byway of example, the invention has been applied to a flyback powerconversion system. But it would be recognized that the invention has amuch broader range of applicability.

FIG. 3 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation. The power conversion system 300includes a primary winding 310, a secondary winding 312, an auxiliarywinding 314, a power switch 320, a current sensing resistor 330, anequivalent resistor 340 for an output cable, resistors 350 and 352, arectifying diode 360, and a controller 370. The controller 370 includesa sampling component 302, a demagnetization detector 304, a capacitor306, a switch 307, a reference-signal generator 308, an oscillator 316,an AND gate 318, a driving component 322, an OR gate 324, comparators326 and 328, a flip-flop component 336, a leading edge blanking (LEB)component 386, resistors 384 and 388, an error amplifier 390, amodulation component 392, and a constant-current (CC) component 394. Forexample, the power switch 320 is a bipolar transistor. In anotherexample, the power switch 320 is a MOS transistor. In yet anotherexample, the controller 370 includes terminals 372, 374, 376, 378 and380.

For example, the auxiliary winding 314 is magnetically coupled to thesecondary winding 312, which, with one or more other components,generates an output voltage 393. In another example, information relatedto the output voltage is processed by a voltage divider of the resistors350 and 352, and is used to generate a feedback voltage 354, which isreceived by the terminal 372 (e.g., terminal FB) of the controller 370.In yet another example, the sampling component 302 samples the feedbackvoltage 354 and the sampled signal is held at the capacitor 306. As anexample, the error amplifier 390 compares the sampled-and-held voltage362 with a reference signal 364 generated by the reference-signalgenerator 308, and outputs a comparison signal 366 associated with theerror of the sampled-and-held voltage 362 with respect to the referencesignal 364. As another example, the comparison signal 366 is received bythe modulation component 392 which receives a clock signal 368 from theoscillator 316 and outputs a modulation signal 356 (e.g., CV_ctrl). Forexample, the comparison signal 366 is used to control the pulse widthfor pulse-width modulation (PWM) and/or the switching frequency forpulse-frequency modulation (PFM) in order to regulate the output voltagein the constant voltage mode. In another example, the demagnetizationdetector 304 determines the duration of a demagnetization period basedon the feedback voltage 354 and outputs a detection signal 358 to theconstant-current component 394 which generates a signal 346 (e.g.,CC_ctrl). In yet another example, both the modulation signal 356 and thesignal 346 are received by the AND gate 318 to affect the flip-flopcomponent 336 and in turn the driving component 322. In yet anotherexample, the driving component 322 outputs a driving signal 348 throughthe terminal 376 to affect the status of the switch 320. In yet anotherexample, a primary current 396 flowing through the primary winding 310is sensed using the resistor 330, and a current-sensing signal 342 isgenerated through the LEB component 386 and received by the comparators326 and 328. In yet another example, the comparator 326 and thecomparator 328 output comparison signals 334 and 338 respectively, tothe OR gate 324 to affect the flip-flop component 336.

As an example, when the sampled-and-held voltage 362 is smaller than thereference signal 364 in magnitude, the error amplifier 390 outputs thecomparison signal 366 at a logic high level. The power conversion system300 operates in the constant-current mode, in some embodiments.

FIG. 4 is a simplified diagram showing at least certain components ofthe constant-current component 394 as part of the power conversionsystem 300. The constant-current component 394 includes a NOT gate 402,current sources 404 and 406, a switch 408, a capacitor 414, a comparator410 and a reference-signal generator 412.

For example, when the detection signal 358 is at a logic low level, theswitch 408 is open (e.g., being turned off) and the switch 416 is closed(e.g., being turned on). In another example, the current source 404provides a current 418 (e.g., I₀) to charge the capacitor 414, and inresponse a signal 420 increases in magnitude. As an example, when thedetection signal 358 is at a logic high level, the switch 416 is open(e.g., being turned off) and the switch 408 is closed (e.g., beingturned on). As another example, the capacitor 414 is discharged throughthe current source 406 which provides a current 424 (e.g., I₁), and thesignal 420 decreases in magnitude. For example, the comparator 410receives the signal 420 and a reference signal 422 generated by thereference-signal generator 412 and outputs the signal 346.

FIG. 5 is a simplified diagram showing at least certain components ofthe demagnetization detector 304 as part of the power conversion system300. The demagnetization detector 304 includes a comparator 502, areference-signal generator 504, flip-flop components 506 and 510, NOTgates 508 and 512, and an AND gate 514. For example, the comparator 502compares the feedback signal 354 with a reference signal 516 (e.g., 0.1V) generated by the reference-signal generator 504, and outputs acomparison signal 518 which is received by the flip-flop components 506and 510. In another example, the NOT gate 508 receives the modulationsignal 356 and outputs a signal 520 to the flip-flop components 506 and510. In yet another example, the AND gate 514 receives a signal 522 fromthe flip-flop component 506 and a signal 525 from the NOT gate 512 andoutputs the detection signal 358.

FIG. 6 is a simplified timing diagram for the power conversion system300. The waveform 602 represents the feedback voltage 354 as a functionof time, the waveform 604 represents the detection signal 358 as afunction of time, and the waveform 606 represents the signal 420 as afunction of time. The waveform 608 represents the signal 346 as afunction of time, the waveform 610 represents the signal 348 as afunction of time, and the waveform 612 represents the current-sensingsignal 342 as a function of time.

Four time periods are shown in FIG. 6. A switching period includes anon-time period T_(on) and an off-time period T_(off) and corresponds toa modulation frequency. The off-time period T_(off) includes ademagnetization period T_(demag). The on-time period starts at time t₀and ends at time t₁, the demagnetization period starts at the time t₁and ends at time t₂, and the off-time period starts at the time t₁ andends at time t₃. For example, t₀≦t₁≦t₂≦t₃.

For example, at the beginning of the on-time period T_(on) (e.g., att₀), the signal 348 changes from a logic low level to a logic high level(e.g., as shown by the waveform 610), and in response the switch 320 isclosed (e.g., being turned on). In another example, the transformerincluding the primary winding 310 and the secondary winding 312 storesenergy, and the primary current 396 increases in magnitude (e.g.,linearly). In yet another example, the current-sensing signal 342increases in magnitude (e.g., as shown by the waveform 612). As anexample, when the current-sensing signal 342 reaches a threshold voltage332 (e.g., V_(thocp)), the comparator 326 changes the comparison signal334 in order to turn off the switch 320. As another example, during theon-time period, the detection signal 358 (e.g., Demag) keeps at a logiclow level (e.g., as shown by the waveform 604). As yet another example,the switch 408 is open (e.g., being turned off) and the switch 416 isclosed (e.g., being turned on). As yet another example, the capacitor414 is charged (e.g., at I₀), and the signal 420 increases in magnitude(e.g., linearly) as shown by the waveform 606.

In one example, at the beginning of the demagnetization period T_(demag)(e.g., at t₁), the signal 348 changes from the logic high level to thelogic low level (e.g., as shown by the waveform 610), and in responsethe switch 320 is opened (e.g., being turned off). In another example,the energy stored in the transformer is released to the output terminal,and the demagnetization process begins. In yet another example, asecondary current 397 that flows through the secondary winding 312decreases in magnitude (e.g., linearly). In yet another example, avoltage 395 at the auxiliary winding 314 maps the output voltage 393,and the feedback voltage 354 is generated through the voltage dividerincluding the resistors 350 and 352. As an example, when the secondarycurrent decreases to a low magnitude (e.g., 0), the demagnetizationprocess ends. As another example, the transformer including the primarywinding 310 and the secondary winding 312 enters a resonant status. Asyet another example, a voltage 395 at the auxiliary winding 314 has anapproximate sinusoidal waveform. In an example, during thedemagnetization period, the detection signal 358 (e.g., Demag) keeps ata logic high level (e.g., as shown by the waveform 604). In yet anotherexample, the switch 416 is opened (e.g., being turned off) and theswitch 408 is closed (e.g., being turned on). In yet another example,the capacitor 414 is discharged (e.g., at I₁), and the signal 420decreases in magnitude (e.g., linearly) as shown by the waveform 606. Inyet another example, if the feedback voltage 354 becomes larger than thereference signal 516 (e.g., 0.1 V) in magnitude, it is determined thatthe demagnetization process has begun. In yet another example, if thefeedback voltage 354 becomes smaller than the reference signal 516(e.g., 0.1 V) in magnitude, it is determined that the demagnetizationprocess has ended.

As one example, after the demagnetization process ends (e.g., at t₂),the detection signal 358 changes from the logic high level to the logiclow level (e.g., as shown by the waveform 604). As another example, theswitch 408 is open (e.g., being turned off) and the switch 416 is closed(e.g., being turned on). As yet another example, the capacitor 414 ischarged again, and the signal 420 increases in magnitude (e.g.,linearly) again as shown by the waveform 606. As yet another example,when the signal 420 becomes larger than a threshold voltage 614 (e.g.,the reference signal 422) in magnitude (e.g., at t₃), the comparator 410changes the signal 346 (e.g., CC_ctrl) from the logic low level to thelogic high level (e.g., as shown by the waveform 608). As yet anotherexample, in response to the signal 346 being at the logic high level,the driving component 322 changes the signal 348 from the logic lowlevel to the logic high level (e.g., at t₃ as shown by the waveform610).

For example, the switching period is determined as follows:

$\begin{matrix}\begin{matrix}{T_{s} = {\frac{I_{0} + I_{1}}{I_{1}}T_{demag}}} \\{= {KT}_{demag}}\end{matrix} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$where I₀ represents the current 418, and I₁ represents the current 424.

The peak value of the primary current 396 is determined as follows:

$\begin{matrix}{I_{p} = \frac{V_{thocp}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$where V_(thocp) represents the threshold voltage 332, and R_(s)represents the resistance of the resistor 330.

Assuming the transmission efficiency of the transformer is 100%, theoutput current is determined as follows:

$\begin{matrix}{I_{out} = \frac{\frac{1}{2}{NI}_{p}T_{demag}}{T_{s}}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

where N represents a turns ratio between the primary winding 310 and thesecondary winding 312.

According to Equations 7-9, the output current is determined as follows:

$\begin{matrix}{I_{out} = {\frac{1}{2}N\frac{V_{thocp}}{K\; R_{s}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$where K is larger than 1. According to Equation 10, the output currentmay be regulated to be approximately constant, in some embodiments.

The operating frequency in the constant-current mode can be determinedas follows:

$\begin{matrix}{F_{cc} = \frac{1}{{KT}_{demag}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$where F_(cc) represents the operating frequency in the constant-currentmode.

The duration of the demagnetization period can be determined as follows:

$\begin{matrix}{T_{demag} = {\frac{N^{2}L_{m}I_{p}}{V_{out} + V_{d}} = {\frac{N^{2}L_{m}}{V_{out} + V_{d}}\frac{V_{thocp}}{R_{s}}}}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$where L_(m) represents the inductance of the primary winding 310, andV_(d) represents a forward voltage drop of the diode 360.

According to Equations 11-12, the operating frequency in theconstant-current mode can be determined as follows:

$\begin{matrix}{F_{cc} = \frac{\left( {V_{out} + V_{d}} \right)R_{s}}{{KN}^{2}L_{m}V_{thocp}}} & \left( {{Equation}\mspace{14mu} 13} \right)\end{matrix}$

FIG. 7(a) is a simplified diagram showing a relationship between theoperating frequency and the output voltage 393 of the power conversionsystem 300 in the constant-current mode under normal operations, andFIG. 7(b) is a simplified diagram showing a relationship between theduration of the demagnetization period and the output voltage 393 of thepower conversion system 300 in the constant-current mode under normaloperations.

For example, if the threshold voltage 332 is approximately constant, theoperating frequency (e.g., F_(cc)) in the constant-current mode isproportional to the output voltage 393 (e.g., as shown by a waveform 702in FIG. 7(a)), and the duration of the demagnetization period in theconstant-current mode is inversely proportional to the output voltage393 (e.g., as shown by a waveform 704 in FIG. 7(b)). In another example,the shaded area A between dashed lines 706 and 708 in FIG. 7(a)indicates the variation of the operating frequency in normal operationsconsidering the inductance of the transformer including the primarywinding 310 varies. In yet another example, the shaded area B betweendashed lines 709 and 710 in FIG. 7(b) indicates the variation of theduration of the demagnetization period in normal operations consideringthe inductance of the transformer including the primary winding 310varies.

As shown in FIG. 7(b), the duration of the demagnetization period withrespect to the output voltage 393 changes in a small area (e.g., theshaded area B) under normal operations, in some embodiments. Forexample, when the duration of the demagnetization period goes beyond theshaded area B, the power conversion system 300 may be deemed as notunder normal operations. Hence, in another example, the power conversionsystem 300 needs to be protected against certain abnormal operations.

FIG. 8 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Thepower conversion system 900 includes a primary winding 910, a secondarywinding 912, an auxiliary winding 914, a power switch 920, a currentsensing resistor 930, an equivalent resistor 940 for an output cable,resistors 950 and 952, a rectifying diode 960, and a controller 970. Thecontroller 970 includes a protection component 901, a sampling component902, a demagnetization detector 904, a capacitor 906, a switch 907, areference-signal generator 908, an oscillator 916, an AND gate 918, adriving component 922, an OR gate 924, comparators 926 and 928, aflip-flop component 936, a leading edge blanking (LEB) component 986,resistors 984 and 988, an error amplifier 990, a modulation component992, and a constant-current (CC) component 994. For example, the powerswitch 920 is a bipolar transistor. In another example, the power switch920 is a MOS transistor. In yet another example, the controller 970includes terminals 972, 974, 976, 978 and 980.

According to one embodiment, the auxiliary winding 914 is magneticallycoupled to the secondary winding 912, which, with one or more othercomponents, generates an output voltage 993. For example, informationrelated to the output voltage is processed by a voltage divider of theresistors 950 and 952, and is used to generate a feedback voltage 954,which is received by the terminal 972 (e.g., terminal FB) of thecontroller 970. In another example, the sampling component 902 samplesthe feedback voltage 954 and the sampled signal is held at the capacitor906. In yet another example, the sampling component 902 samples thefeedback voltage 954 at the middle point of the demagnetization period.

According to another embodiment, the error amplifier 990 compares thesampled-and-held voltage 962 with a reference signal 964 generated bythe reference-signal generator 908, and outputs a comparison signal 966associated with the error of the sampled-and-held voltage 962 withrespect to the reference signal 964. For example, the comparison signal966 is received by the modulation component 992 which receives a clocksignal 968 from the oscillator 916 and outputs a modulation signal 956(e.g., CV_ctrl). In another example, the comparison signal 966 is usedto control the pulse width for pulse-width modulation (PWM) and/or theswitching frequency for pulse-frequency modulation (PFM) in order toregulate the output voltage in the constant voltage mode. In yet anotherexample, when the sampled-and-held voltage 962 is smaller than thereference signal 964 in magnitude, the error amplifier 990 outputs thecomparison signal 966 at a logic high level in order to operate thepower conversion system 900 operates in the constant-current mode. Inyet another example, the demagnetization detector 904 determines theduration of a demagnetization period based on the feedback voltage 954and outputs a detection signal 958 to the constant-current component 994which generates a signal 946 (e.g., CC_ctrl). In yet another example,both the modulation signal 956 and the signal 946 are received by theAND gate 918 to affect the flip-flop component 936.

According to yet another embodiment, the driving component 922 outputs adriving signal 948 through the terminal 976 to affect the status of theswitch 920. For example, a primary current 996 flowing through theprimary winding 910 is sensed using the resistor 930, and acurrent-sensing signal 942 is generated through the LEB component 986and received by the comparators 926 and 928. In another example, thecomparator 926 and the comparator 928 output comparison signals 934 and938 respectively, to the OR gate 924 to affect the flip-flop component936. In yet another example, the protection component 901 receives thefeedback voltage 954 and outputs a signal 903 (e.g., fault) to theflip-flop component 936. In yet another example, the driving component922 receives a signal 905 from the flip-flop component and the signal903 and outputs a driving signal 948 to affect the switch 920.

FIG. 9 is a simplified diagram showing certain components of theprotection component 901 as part of the power conversion system 900according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The protection component901 includes an output-voltage detector 1002, a voltage-controlled-timercomponent 1004, and a timer comparator 1006.

According to one embodiment, the output-voltage detector 1002 receivesthe feedback voltage 954 and outputs a signal 1008 (e.g., V_(sap)). Forexample, the signal 1008 (e.g., V_(sap)) is associated with (e.g.,approximately proportional to) the output voltage 993. In anotherexample, the voltage-controlled-timer component 1004 receives the signal1008 and outputs a signal 1010. In yet another example, the signal 1010corresponds to a reference duration (e.g., T_(ref)) which has a waveformwith respect to the output voltage 993. In yet another example, thetimer comparator 1006 compares the detection signal 958 which indicatesthe duration of the demagnetization period of the power conversionsystem 900 and the signal 1010 and outputs the signal 903 (e.g., fault).In yet another example, if the reference duration (e.g., T_(ref)) issmaller than the duration of the demagnetization period of the powerconversion system 900, the timer comparator 1006 outputs the signal 903(e.g., fault) at the logic low level which indicates that the powerconversion system 900 is under normal operations. In yet anotherexample, if the reference duration (e.g., T_(ref)) is larger than theduration of the demagnetization period of the power conversion system900, the timer comparator 1006 outputs the signal 903 (e.g., fault) atthe logic high level which indicates that the power conversion system900 is not under normal operations.

$\begin{matrix}{V_{sap} = {\frac{R_{2}}{R_{1} + R_{2}}\left( {V_{out} + V_{d}} \right)}} & \left( {{Equation}\mspace{14mu} 14} \right)\end{matrix}$where V_(sap) represents the signal 1088, V_(d) represents a forwardvoltage drop of the diode 960, R₁ represents a resistance of theresistor 950, and R₂ represents a resistance of the resistor 952. In yetanother example, the signal 1088 represents the output voltage 993 undernormal operations.

According to another embodiment, the reference duration (e.g., T_(ref))is determined as follows:

$\begin{matrix}{T_{ref} = {\frac{1}{M}\frac{N^{2}L_{m}}{V_{out} + V_{d}}\frac{V_{thocp}}{R_{s}}}} & \left( {{Equation}\mspace{14mu} 15} \right)\end{matrix}$where N represents a turns ratio between the primary winding 910 and thesecondary winding 912, V_(thocp) represents a threshold voltage 932, andR_(s) represents the resistance of the resistor 930. In addition, L_(m)represents the inductance of the primary winding 910, V_(out) representsthe output voltage 993, V_(d) represents a forward voltage drop of thediode 960, and M is a constant (e.g., larger than 1). For example, M isin the range of 1.4˜2. In another example, V_(thocp) has a fixedmagnitude.

FIG. 10 is a simplified diagram showing a relationship between theduration of the demagnetization period and the signal 1008 of the powerconversion system 900 under normal operations, and under certainabnormal operations against which the power conversion system 900 isprotected, according to one embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The waveform 1202represents a relationship between the duration of the demagnetizationperiod and the signal 1008 (e.g., V_(sap)), and the waveform 1204represents a relationship between the reference duration T_(ref) and thesignal 1008 (e.g., V_(sap)) of the power conversion system 900. Forexample, under normal operations, the signal 1008 (e.g., V_(sap))represents the output voltage 993 of the power conversion system 900.

As shown in FIG. 10, the duration of the demagnetization period withrespect to the signal 1008 changes in a small area (e.g., the shadedarea E between dashed lines 1206 and 1208) under normal operations, insome embodiments. For example, when the duration of the demagnetizationperiod changes in the shaded area E, the signal 948 is output as amodulation signal to turn on and turn off the switch 920 within aswitching period. In another example, when the duration of thedemagnetization period with respect to the signal 1008 goes into anothershaded area F below the waveform 1204, the signal 1008 does notrepresent the output voltage 993, and the power conversion system 900 isunder certain abnormal operations against which the power conversionsystem 900 is protected. In another example, the switch 920 is opened(e.g., being turned off) to protect the power conversion system 900. Inyet another example, the waveform 1204 is parallel to the waveform 1202.

FIG. 11 is a simplified diagram showing certain components of theprotection component 901 as part of the power conversion system 900according to another embodiment of the present invention. This diagramis merely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The protection component901 includes an output-voltage detector 1102, and a timer-and-comparatorcomponent 1104. The output-voltage detector 1102 includes a switch 1103,a sampling component 1109, and a capacitor 1106. Thetimer-and-comparator component 1104 includes an amplifier 1108, a NOTgate 1110, transistors 1114, 1116, 1118, 1120, 1122, a resistor 1112, acapacitor 1124, a reference-signal generator 1128, a comparator 1126,and a cycle-debounce component 1130. For example, the output-voltagedetector 1102 is the output-voltage detector 1002, thetimer-and-comparator component 1104 is a combination of thevoltage-controlled-timer component 1004 and the timer comparator 1006,and the signal 1188 is the signal 1008.

According to one embodiment, the sampling component 1109 samples thefeedback voltage 954 and the sampled signal is held at the capacitor1106. For example, the output-voltage detector 1102 outputs asampled-and-held signal 1188 (e.g., V_(sap)) to the timer-and-comparatorcomponent 1104. In another example, the sampled-and-held signal 1188 isdetermined as follows:

$\begin{matrix}{V_{sap} = {\frac{R_{2}}{R_{1} + R_{2}}\left( {V_{out} + V_{d}} \right)}} & \left( {{Equation}\mspace{14mu} 16} \right)\end{matrix}$where V_(sap) represents the signal 1188, V_(d) represents a forwardvoltage drop of the diode 960, R₁ represents a resistance of theresistor 950, and R₂ represents a resistance of the resistor 952. In yetanother example, the sampling component 1109 samples the feedbackvoltage 954 at a time that is no earlier than the middle point of thedemagnetization period but is no later than a point that is ⅚ of thedemagnetization period away from the beginning of the demagnetizationperiod.

According to another embodiment, the amplifier 1108 receives the signal1188 and outputs a signal 1132 to the transistor 1118 so that a current1134 flows through the transistor 1114, the transistor 1118 and theresistor 1112. For example, during the demagnetization process, the NOTgate 1110 receives the detection signal 958 at a logic high level. Inanother example, the transistor 1120 is turned on and the transistor1122 is turned off. In yet another example, a current 1138 flows throughthe transistors 1116 and 1120 to charge the capacitor 1124, and a signal1136 increases in magnitude. In yet another example, the comparator 1126compares the signal 1136 and a reference signal 1140 from thereference-signal generator 1128 and outputs a comparison signal 1142. Ifthe power conversion system 900 operates under normal operations, thesignal 1136 is larger than the reference signal 1140 in magnitude, andthe comparator 1126 outputs the comparison signal 1142 at the logic lowlevel, in some embodiments. For example, the signal 903 is at the logiclow level. If the power conversion system 900 does not operate undernormal operations, the signal 1136 is smaller than the reference signal1140 in magnitude, and the comparator 1126 outputs the comparison signal1142 at the logic high level, in some embodiments. For example, thesignal 903 is at the logic high level. In another example, in responseto the signal 903 being at the logic high level, the switch 920 isopened (e.g., be turned off) for a time period longer than at least aswitching period (e.g., without any modulation) to protect the system900. In yet another example, in response to the signal 903 being at thelogic high level, the system 900 is shut down and the switch 920 keepsopen. In yet another example, after being shut down, the system 900restarts (e.g., automatically or manually) and starts modulation again.In yet another example, the switch 920 is closed (e.g., being turned on)and opened (e.g., being turned off) at a modulation frequency again. Incertain embodiments, the cycle-debounce component 1130 is omitted, andthe signal 903 is the same as the signal 1142.

According to yet another embodiment, a reference duration T_(ref1)corresponds to the reference signal 1140 is determined as follows:

$\begin{matrix}{T_{{ref}\; 1} = {\frac{R_{0}C_{1}V_{{ref}\; 2}}{V_{sap}} = {R_{0}C_{1}\frac{R_{1} + R_{2}}{R_{2}}\frac{V_{{ref}\; 2}}{V_{out} + V_{d}}}}} & \left( {{Equation}\mspace{14mu} 17} \right)\end{matrix}$where R₀ represents a resistance of the resistor 1112, C₁ represents acapacitance of the capacitor 1124, and V_(ref2) represents the referencesignal 1140.

In yet another example, according to Equation 15, the reference durationT_(ref1) is set to be equal to T_(ref):

$\begin{matrix}{T_{{ref}\; 1} = {T_{ref} = {\frac{1}{M}\frac{N^{2}L_{m}}{V_{out} + V_{d}}\frac{V_{thocp}}{R_{s}}}}} & \left( {{Equation}\mspace{14mu} 18} \right)\end{matrix}$

According to Equations 17-18, the constant M is determined as follows,in some embodiments:

$\begin{matrix}{M = {\frac{N^{2}L_{m}V_{thocp}}{R_{s}V_{{ref}\; 2}}\frac{R_{2}}{\left( {R_{1} + R_{2}} \right)}\frac{1}{R_{0}C_{1}}}} & \left( {{Equation}\mspace{14mu} 19} \right)\end{matrix}$For example, if N, L_(m), V_(thocp), V_(ref2), R_(s), R₁, R₂, R₀, and C₁are chosen properly, the constant M is larger than 1 so that thereference duration T_(ref1) has a waveform with respect to the outputvoltage 993 similar to the waveform 1204 as shown in FIG. 10.

As discussed above and further emphasized here, FIGS. 8, 9 and 11 aremerely examples, which should not unduly limit the scope of the claims.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. In one embodiment, the protectioncomponent 901 receives the sampled-and-held voltage 962 instead of thefeedback voltage 954, and the protection component 901 does not includethe output-voltage detector 1002 or the output-voltage detector 1102.For example, the switch 1103 is the switch 907, the capacitor 1106 isthe capacitor 906, and the sampling component 1109 is the samplingcomponent 902. In another example, the sampling component 902 samplesthe feedback voltage 954 at the middle point of the demagnetizationperiod. In another embodiment, the threshold voltage 932 does not have afixed magnitude, as shown in FIG. 12 and FIG. 13.

As discussed above and further emphasized here, FIG. 10 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the waveform 1202 and the waveform 1204are affected by the current-sensing signal 996. Therefore, theprotection component receives the current-sensing signal as an input, asshown in FIG. 12.

FIG. 12 is a simplified diagram showing a power conversion system withprimary-side sensing and regulation according to another embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.The power conversion system 1600 includes a primary winding 1610, asecondary winding 1612, an auxiliary winding 1614, a power switch 1620,a current sensing resistor 1630, an equivalent resistor 1640 for anoutput cable, resistors 1650 and 1652, a rectifying diode 1660, and acontroller 1670. The controller 1670 includes a protection component1601, a sampling component 1602, a demagnetization detector 1604, acapacitor 1606, a switch 1607, a reference-signal generator 1608, anoscillator 1616, an AND gate 1618, a driving component 1622, an OR gate1624, comparators 1626 and 1628, a flip-flop component 1636, a leadingedge blanking (LEB) component 1686, resistors 1684 and 1688, an erroramplifier 1690, a modulation component 1692, and a constant-current (CC)component 1694. For example, the power switch 1620 is a bipolartransistor. In another example, the power switch 1620 is a MOStransistor. In yet another example, the controller 1670 includesterminals 1672, 1674, 1676, 1678 and 1680.

According to one embodiment, the auxiliary winding 1614 is magneticallycoupled to the secondary winding 1612, which, with one or more othercomponents, generates an output voltage 1693. For example, informationrelated to the output voltage is processed by a voltage divider of theresistors 1650 and 1652, and is used to generate a feedback voltage1654, which is received by the terminal 1672 (e.g., terminal FB) of thecontroller 1670. In another example, the sampling component 1602 samplesthe feedback voltage 1654 and the sampled signal is held at thecapacitor 1606. In yet another example, the sampling component 1602samples the feedback voltage 1654 at the middle point of thedemagnetization period.

According to another embodiment, the error amplifier 1690 compares thesampled-and-held voltage 1662 with a reference signal 1664 generated bythe reference-signal generator 1608, and outputs a comparison signal1666 associated with the error of the sampled-and-held voltage 1662 withrespect to the reference signal 1664. For example, the comparison signal1666 is received by the modulation component 1692 which receives a clocksignal 1668 from the oscillator 1616 and outputs a modulation signal1656 (e.g., CV_ctrl). In another example, the comparison signal 1666 isused to control the pulse width for pulse-width modulation (PWM) and/orthe switching frequency for pulse-frequency modulation (PFM) in order toregulate the output voltage in the constant voltage mode. In yet anotherexample, when the sampled-and-held voltage 1662 is smaller than thereference signal 1664 in magnitude, the error amplifier 1690 outputs thecomparison signal 1666 at a logic high level in order to operate thepower conversion system 1600 operates in the constant-current mode. Inyet another example, the demagnetization detector 1604 determines theduration of a demagnetization period based on the feedback voltage 1654and outputs a detection signal 1658 to the constant-current component1694 which generates a signal 1646 (e.g., CC_ctrl). In yet anotherexample, both the modulation signal 1656 and the signal 1646 arereceived by the AND gate 1618 to affect the flip-flop component 1636.

According to yet another embodiment, the driving component 1622 outputsa driving signal 1648 through the terminal 1676 to affect the status ofthe switch 1620. For example, a primary current 1696 flowing through theprimary winding 1610 is sensed using the resistor 1630, and acurrent-sensing signal 1642 is generated through the LEB component 1686and received by the comparators 1626 and 1628. In another example, thecomparator 1626 and the comparator 1628 output comparison signals 1634and 1638 respectively, to the OR gate 1624 to affect the flip-flopcomponent 1636. In yet another example, the protection component 1601receives the feedback voltage 1654 and the current-sensing signal 1642and outputs a signal 1603 (e.g., fault) to the flip-flop component 1636.In yet another example, the driving component 1622 receives a signal1605 from the flip-flop component and the signal 1603 and outputs adriving signal 1648 to affect the switch 1620.

FIG. 13 is a simplified diagram showing certain components of theprotection component 1601 as part of the power conversion system 1600according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The protection component1601 includes an output-voltage detector 1302, avoltage-controlled-timer component 1304, a timer comparator 1306, and apeak-current detector 1308.

According to one embodiment, the output-voltage detector 1302 receivesthe feedback voltage 1654 and outputs a signal 1310 (e.g., V_(sap)). Forexample, the signal 1310 (e.g., V_(sap)) is associated with (e.g.,approximately proportional to) the output voltage 1693. In anotherexample, the peak-current detector 1308 receives the current-sensingsignal 1642 and outputs the threshold voltage 1632. In yet anotherexample, the voltage-controlled-timer component 1304 receives the signal1310 and the threshold voltage 1632 and outputs a signal 1312. In yetanother example, the signal 1312 corresponds to a reference duration(e.g., T_(ref3)). In yet another example, the timer comparator 1306compares the detection signal 1658 which indicates the duration of thedemagnetization period of the power conversion system 1600 and thesignal 1312 and outputs the signal 1603 (e.g., fault). In yet anotherexample, if the reference duration (e.g., T_(ref3)) is smaller than theduration of the demagnetization period of the power conversion system1600, the timer comparator 1306 outputs the signal 1603 (e.g., fault) atthe logic low level which indicates that the power conversion system1600 is under normal operations. In yet another example, if thereference duration (e.g., T_(ref3)) is larger than the duration of thedemagnetization period of the power conversion system 1600, the timercomparator 1306 outputs the signal 1603 (e.g., fault) at the logic highlevel which indicates that the power conversion system 1600 is not undernormal operations.

According to another embodiment, the reference duration (e.g., T_(ref3))is determined as follows:

$\begin{matrix}{T_{{ref}\; 3} = {\frac{1}{M}\frac{N^{2}L_{m}}{V_{out} + V_{d}}\frac{V_{thocp}}{R_{s}}}} & \left( {{Equation}\mspace{14mu} 20} \right)\end{matrix}$where N represents a turns ratio between the primary winding 1610 andthe secondary winding 1612, V_(thocp) represents a threshold voltage1632, and R_(s) represents the resistance of the resistor 1630. Inaddition, L_(m) represents the inductance of the primary winding 1610,V_(out) represents the output voltage 1693, V_(d) represents a forwardvoltage drop of the diode 1660, and M is a constant (e.g., larger than1). For example, M is in the range of 1.4˜2. In another example,V_(thocp) has a variable magnitude.

FIG. 14 is a simplified diagram showing a relationship between theduration of the demagnetization period and the signal 1310 of the powerconversion system 1600 under normal operations, and under certainabnormal operations against which the power conversion system 1600 isprotected, according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The waveform 1702represents a relationship between the duration of the demagnetizationperiod and the signal 1310 (e.g., V_(sap)), and the waveform 1704represents a relationship between the reference duration T_(ref3) andthe signal 1310 of the power conversion system 1600. For example, undernormal operations, the signal 1310 (e.g., V_(sap)) represents the outputvoltage 1693 of the power conversion system 1600.

As shown in FIG. 14, the duration of the demagnetization period withrespect to the signal 1310 changes in a small area (e.g., the shadedarea G between dashed lines 1706 and 1708) under normal operations, insome embodiments. For example, when the duration of the demagnetizationperiod changes in the shaded area G, the signal 1648 is output as amodulation signal to turn on and turn off the switch 1620 within aswitching period. In another example, when the duration of thedemagnetization period with respect to the signal 1310 goes into anothershaded area H below the waveform 1704, the signal 1310 does notrepresent the output voltage 1693, and the power conversion system 1600is under certain abnormal operations against which the power conversionsystem 1600 is protected. In another example, the switch 1620 is opened(e.g., being turned off) to protect the power conversion system 1600. Inyet another example, the waveform 1704 is parallel to the waveform 1702.In yet another example, the waveforms 1702 and 1704 both change with thethreshold voltage 1632.

FIG. 15 is a simplified diagram showing certain components of theprotection component 1601 as part of the power conversion system 1600according to another embodiment of the present invention. This diagramis merely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The protection component1601 includes an output-voltage detector 1402, a timer-and-comparatorcomponent 1404, and a peak-current detector 1498. The output-voltagedetector 1402 includes a switch 1403, a sampling component 1409, and acapacitor 1406. The timer-and-comparator component 1404 includes anamplifier 1408, a NOT gate 1410, transistors 1414, 1416, 1418, 1420,1422, a resistor 1412, a capacitor 1424, a comparator 1426, and acycle-debounce component 1430. For example, the output-voltage detector1402 is the output-voltage detector 1302, the peak-current detector 1498is the peak-current detector 1308, the timer-and-comparator component1404 is a combination of the voltage-controlled-timer component 1304 andthe timer comparator 1306, and the signal 1488 is the signal 1310. Inanother example, the switch 1403, the sampling component 1409 and thecapacitor 1406 are the same as the switch 1607, the sampling component1602, and the capacitor 1606, respectively. In yet another example, acapacitor 1428 is included in the peak-current detector 1498. In yetanother example, the capacitor 1428 is included in thetimer-and-comparator component 1404.

According to one embodiment, the sampling component 1409 samples thefeedback voltage 1654 and the sampled signal is held at the capacitor1406. For example, the output-voltage detector 1402 outputs asampled-and-held signal 1488 (e.g., V_(sap)) to the timer-and-comparatorcomponent 1404. In another example, the sampled-and-held signal 1488 isdetermined as follows:

$\begin{matrix}{V_{sap} = {\frac{R_{2}}{R_{1} + R_{2}}\left( {V_{out} + V_{d}} \right)}} & \left( {{Equation}\mspace{14mu} 21} \right)\end{matrix}$where V_(sap) represents the signal 1488, V_(d) represents a forwardvoltage drop of the diode 1660, R₁ represents a resistance of theresistor 1650, and R₂ represents a resistance of the resistor 1652. Inyet another example, the sampling component 1409 samples the feedbackvoltage 1654 at a time that is no earlier than the middle point of thedemagnetization period but is no later than a point that is ⅚ of thedemagnetization period away from the beginning of the demagnetizationperiod.

According to another embodiment, the amplifier 1408 receives the signal1488 and outputs a signal 1432 to the transistor 1418 so that a current1434 flows through the transistor 1414, the transistor 1418 and theresistor 1412. For example, during the demagnetization process, the NOTgate 1410 receives the detection signal 1658 at a logic high level. Inanother example, the transistor 1420 is turned on and the transistor1422 is turned off. In yet another example, a current 1438 flows throughthe transistors 1416 and 1420 to charge the capacitor 1424, and a signal1436 increases in magnitude. In yet another example, the peak-currentdetector 1498 receives the current-sensing signal 1642 and outputs thethreshold voltage 1632. In yet another example, the comparator 1426compares the signal 1436 and the threshold voltage 1632 and outputs acomparison signal 1442. If the power conversion system 1600 operatesunder normal operations, the signal 1436 is larger than the thresholdvoltage 1632 in magnitude, and the comparator 1426 outputs thecomparison signal 1442 at the logic low level, in some embodiments. Forexample, the signal 1603 is at the logic low level. If the powerconversion system 1600 does not operate under normal operations, thesignal 1436 is smaller than the threshold voltage 1632 in magnitude, andthe comparator 1426 outputs the comparison signal 1442 at the logic highlevel, in some embodiments. For example, the signal 1603 is at the logichigh level. In another example, in response to the signal 1603 being atthe logic high level, the switch 1620 is opened (e.g., be turned off)for a time period longer than at least a switching period (e.g., withoutany modulation) to protect the system 1600. In yet another example, inresponse to the signal 1603 being at the logic high level, the system1600 is shut down and, the switch 1620 keeps open. In yet anotherexample, after being shut down, the system 1600 restarts (e.g.,automatically or manually) and starts modulation again. In yet anotherexample, the switch 1620 is closed (e.g., being turned on) and opened(e.g., being turned off) at a modulation frequency again. In certainembodiments, the cycle-debounce component 1430 is omitted, and thesignal 1603 is the same as the signal 1442.

According to yet another embodiment, a reference duration T_(ref4)corresponds to the threshold voltage 1632 is determined as follows:

$\begin{matrix}{T_{{ref}\; 4} = {\frac{R_{0}C_{1}V_{thocp}}{V_{sap}} = {R_{0}C_{1}\frac{R_{1} + R_{2}}{R_{2}}\frac{V_{thocp}}{V_{out} + V_{d}}}}} & \left( {{Equation}\mspace{14mu} 22} \right)\end{matrix}$where R₀ represents a resistance of the resistor 1412, and C₁ representsa capacitance of the capacitor 1424.

In yet another example, according to Equation 20, the reference durationT_(ref4) is set to be equal to T_(ref3):

$\begin{matrix}{T_{{ref}\; 4} = {T_{{ref}\; 3} = {\frac{1}{M}\frac{N^{2}L_{m}}{V_{out} + V_{d}}\frac{V_{thocp}}{R_{s}}}}} & \left( {{Equation}\mspace{14mu} 23} \right)\end{matrix}$

According to Equations 22-23, the constant M is determined as follows,in some embodiments:

$\begin{matrix}{M = {\frac{N^{2}L_{m}}{R_{s}}\frac{R_{2}}{\left( {R_{1} + R_{2}} \right)}\frac{1}{R_{0}C_{1}}}} & \left( {{Equation}\mspace{14mu} 24} \right)\end{matrix}$For example, if N, L_(m), R_(s), R₁, R₂, R₀, and C₁ are chosen properly,the constant M is larger than 1 so that the reference duration T_(ref4)has a waveform with respect to the output voltage 1693 similar to thewaveform 1704 as shown in FIG. 14.

Referring back to FIG. 8, in some embodiments, if the resistor 950 isopen-circuited or if the resistor 952 is short-circuited, thedemagnetization detector 904 cannot detect the duration of thedemagnetization period and the feedback voltage 954 has a low magnitude(e.g., 0). For example, as shown in FIG. 11, the capacitor 1124 is notcharged and the comparator 1126 outputs the comparison signal 1142 atthe logic high level. In another example, the signal 903 is at the logichigh level, and the switch 920 is opened (e.g., being turned off) toprotect the power conversion system 900.

Referring back to FIG. 12, in certain embodiments, if the resistor 1650is open-circuited or if the resistor 1652 is short-circuited, thedemagnetization detector 1604 cannot detect the duration of thedemagnetization period and the feedback voltage 1654 has a low magnitude(e.g., 0). For example, as shown in FIG. 15, the capacitor 1424 is notcharged and the comparator 1426 outputs the comparison signal 1442 atthe logic high level. In another example, the signal 1603 is at thelogic high level, and the switch 1620 is opened (e.g., being turned off)to protect the power conversion system 1600.

As discussed above and further emphasized here, FIGS. 12, 13 and 15 aremerely examples, which should not unduly limit the scope of the claims.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. For example, the protection component1601 receives the sampled-and-held voltage 1662 instead of the feedbackvoltage 1654, and the protection component 1601 does not include theoutput-voltage detector 1302 or the output-voltage detector 1402. Forexample, the switch 1403 is the switch 1607, the capacitor 1406 is thecapacitor 1606, and the sampling component 1409 is the samplingcomponent 1602. In another example, the sampling component 1602 samplesthe feedback voltage 1654 at the middle point of the demagnetizationperiod.

FIG. 16 is a simplified diagram showing certain protection processimplemented by the power conversion system 900 and/or the powerconversion system 1600 according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown inFIG. 16, a power conversion system 1500 includes an auxiliary winding1514, resistors 1550 and 1552, a capacitor 1595, and a diode 1593.

In one embodiment, the system 1500 is the same as the system 900. Forexample, the auxiliary winding 1514 is the same as the auxiliary winding914, and the resistors 1550 and 1552 are the same as the resistors 950and 952 respectively. In some embodiments, if a terminal 1504 (e.g.,T_(aux)) of the auxiliary winding 1514 is open-circuited, thedemagnetization detector 1504 cannot detect the duration of thedemagnetization period and a feedback voltage 1554 (e.g., the feedbackvoltage 954) has a low magnitude (e.g., 0). For example, as shown inFIG. 11, the capacitor 1124 is not charged and the comparator 1126outputs the comparison signal 1142 at the logic high level. In anotherexample, the signal 903 is at the logic high level, and the switch 920is opened (e.g., being turned off) to protect the power conversionsystem 900. In certain embodiments, if a terminal 1506 (T_(ground)) ofthe auxiliary winding 1514 is open-circuited, a parasitic capacitor 1502exists. For example, as shown in FIG. 8, the duration of thedemagnetization period detected by the demagnetization detector 904 isclose to the actual duration of the demagnetization period. In anotherexample, as shown in FIG. 11, the sampled-and-held signal 1188 relatedto the feedback voltage 954 has a smaller magnitude than the magnitudeunder normal operations, the signal 1136 is smaller than the referencesignal 1140 in magnitude, and the comparator 1126 outputs the comparisonsignal 1142 at the logic high level. In yet another example, the signal903 is at the logic high level, and the switch 920 is opened (e.g.,being turned off) to protect the power conversion system 900.

In another embodiment, the system 1500 is the system 1600. For example,the auxiliary winding 1514 is the same as the auxiliary winding 1614,and the resistors 1550 and 1552 are the same as the resistors 1650 and1652 respectively. In some embodiments, if a terminal 1504 (e.g.,T_(aux)) of the auxiliary winding 1514 is open-circuited, thedemagnetization detector 1504 cannot detect the duration of thedemagnetization period and a feedback voltage 1554 (e.g., the feedbackvoltage 1654) has a low magnitude (e.g., 0). For example, as shown inFIG. 15, the capacitor 1424 is not charged and the comparator 1426outputs the comparison signal 1442 at the logic high level. In anotherexample, the signal 1603 is at the logic high level, and the switch 1620is opened (e.g., being turned off) to protect the power conversionsystem 1600. In certain embodiments, if a terminal 1506 (T_(ground)) ofthe auxiliary winding 1514 is open-circuited, a parasitic capacitor 1502exists. For example, as shown in FIG. 12, the duration of thedemagnetization period detected by the demagnetization detector 1604 isclose to the actual duration of the demagnetization period. In anotherexample, as shown in FIG. 14, the sampled-and-held signal 1488 relatedto the feedback voltage 1654 has a smaller magnitude than the magnitudeunder normal operations, the signal 1436 is smaller than the thresholdvoltage 1632 in magnitude, and the comparator 1426 outputs thecomparison signal 1442 at the logic high level. In yet another example,the signal 1603 is at the logic high level, and the switch 1620 isopened (e.g., being turned off) to protect the power conversion system1600.

According to another embodiment, a system controller for protecting apower conversion system includes a protection component and a drivingcomponent. The protection component is configured to receive ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system,process information associated with the demagnetization signal and adetected voltage generated based on at least information associated withthe feedback signal, and generate a protection signal based on at leastinformation associated with the detected voltage and the demagnetizationsignal. The driving component is configured to receive the protectionsignal and output a driving signal to a switch configured to affect aprimary current flowing through a primary winding of the powerconversion system. The detected voltage is related to an output voltageof the power conversion system. The demagnetization signal is related toa demagnetization period of the power conversion system. The protectioncomponent and the driving component are further configured to, if thedetected voltage and the demagnetization signal satisfy one or moreconditions, output the driving signal to cause the switch to open andremain open in order to protect the power conversion system. Forexample, the system controller is implemented according to at least FIG.8, FIG. 9, FIG. 10, and/or FIG. 11.

According to yet another embodiment, a system controller for protectinga power conversion system includes a protection component and a drivingcomponent. The protection component is configured to receive ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system,receive a current-sensing signal associated with a primary currentflowing through a primary winding of the power conversion system,process information associated with the demagnetization signal, thecurrent-sensing signal, and a detected voltage generated based on atleast information associated with the feedback signal, and generate aprotection signal based on at least information associated with thedetected voltage, the demagnetization signal, and the current-sensingsignal. The driving component is configured to receive the protectionsignal and output a driving signal to a switch configured to affect theprimary current flowing through the primary winding. The detectedvoltage is related to an output voltage of the power conversion system.The demagnetization signal is related to a demagnetization period of thepower conversion system. The protection component and the drivingcomponent are further configured to, if the detected voltage, thedemagnetization signal and the current-sensing signal satisfy one ormore conditions, output the driving signal to cause the switch to openand remain open in order to protect the power conversion system. Forexample, the system controller is implemented according to at least FIG.12, FIG. 13, FIG. 14, and/or FIG. 15.

In one embodiment, a method for protecting a power conversion systemincludes, receiving a demagnetization signal generated based on at leastinformation associated with a feedback signal of the power conversionsystem, processing information associated with the demagnetizationsignal and a detected voltage generated based on at least informationassociated with the feedback signal, and generating a protection signalbased on at least information associated with the detected voltage andthe demagnetization signal. The method further includes, receiving theprotection signal, generating a driving signal based on at leastinformation associated with the protection signal, and outputting thedriving signal to a switch configured to affect a primary currentflowing through a primary winding of the power conversion system. Thedetected voltage is related to an output voltage of the power conversionsystem. The demagnetization signal is related to a demagnetizationperiod of the power conversion system. The process for outputting adriving signal to a switch configured to affect a primary currentflowing through a primary winding of the power conversion systemincludes, if the detected voltage and the demagnetization signal satisfyone or more conditions, outputting the driving signal to cause theswitch to open and remain open in order to protect the power conversionsystem. For example, the method is implemented according to at leastFIG. 8, FIG. 9, FIG. 10, and/or FIG. 11.

In another embodiment, a method for protecting a power conversion systemincludes, receiving a demagnetization signal generated based on at leastinformation associated with a feedback signal of the power conversionsystem, receiving a current-sensing signal associated with a primarycurrent flowing through a primary winding of the power conversionsystem, and processing information associated with the demagnetizationsignal, the current-sensing signal, and a detected voltage generatedbased on at least information associated with the feedback signal. Themethod further includes, generating a protection signal based on atleast information associated with the detected voltage, thedemagnetization signal, and the current-sensing signal, receiving theprotection signal, generating a driving signal based on at leastinformation associated with the protection signal, and outputting thedriving signal to a switch configured to affect the primary currentflowing through the primary winding. The detected voltage is related toan output voltage of the power conversion system. The demagnetizationsignal is related to a demagnetization period of the power conversionsystem. The process for outputting a driving signal to a switchconfigured to affect the primary current flowing through the primarywinding includes, if the detected voltage, the demagnetization signaland the current-sensing signal satisfy one or more conditions,outputting the driving signal to cause the switch to open and remainopen in order to protect the power conversion system. For example, themethod is implemented according to at least FIG. 12, FIG. 13, FIG. 14,and/or FIG. 15.

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system controller for protecting a powerconversion system, the system controller comprising: a protectioncomponent configured to receive a demagnetization signal generated basedon at least information associated with a feedback signal of the powerconversion system, process information associated with thedemagnetization signal and a detected voltage generated based on the atleast information associated with the feedback signal, and generate aprotection signal based on at least information associated with thedetected voltage and the demagnetization signal; and a driving componentconfigured to receive the protection signal and output a driving signalto a switch configured to affect a primary current flowing through aprimary winding of the power conversion system; wherein: the detectedvoltage is related to an output voltage of the power conversion system;and the demagnetization signal is related to a demagnetization period ofthe power conversion system; wherein the protection component and thedriving component are further configured to, if the detected voltage andthe demagnetization signal satisfy one or more conditions, output thedriving signal to cause the switch to open and remain open in order toprotect the power conversion system; wherein the protection componentand the driving component are further configured to, in response to atleast the detected voltage and the demagnetization signal not satisfyingthe one or more conditions, output the driving signal as a modulationsignal to turn on and turn off the switch within a switching periodcorresponding to a modulation frequency.
 2. The system controller ofclaim 1 wherein the protection component and the driving component arefurther configured to, if the detected voltage and the demagnetizationsignal satisfy the one or more conditions, shut down the systemcontroller.
 3. The system controller of claim 1 wherein the protectioncomponent and the driving component are further configured to, if thedetected voltage and the demagnetization signal satisfy the one or moreconditions, output the driving signal to open the switch without anymodulation.
 4. The system controller of claim 1 wherein the protectioncomponent and the driving component are further configured to, if thedetected voltage and the demagnetization signal satisfy the one or moreconditions, output the driving signal to open the switch for a period oftime longer than at least the switching period.
 5. The system controllerof claim 1 wherein the protection component is further configured toreceive the detected voltage generated based on at least informationassociated with the feedback signal.
 6. The system controller of claim 1wherein the protection component is further configured to receive thefeedback signal and generate the detected voltage based on at leastinformation associated with the feedback signal.
 7. The systemcontroller of claim 1 wherein the detected voltage and thedemagnetization signal satisfy the one or more conditions if thedemagnetization period is smaller in duration than a threshold timeperiod corresponding to the detected voltage.
 8. The system controllerof claim 7 wherein the duration of the threshold time period isinversely proportional to the detected voltage.
 9. The system controllerof claim 1 wherein the protection component includes a voltage detectorconfigured to receive the feedback signal and generate the detectedvoltage based on the at least information associated with the feedbacksignal.
 10. The system controller of claim 9 wherein the voltagedetector is configured to receive the feedback signal from avoltage-signal generator coupled to an auxiliary winding of the powerconversion system.
 11. The system controller of claim 10 wherein thevoltage-signal generator is configured to generate the feedback signalbased on at least information associated with the output voltage, theoutput voltage being related to a secondary winding of the powerconversion system.
 12. The system controller of claim 11 wherein thesecondary winding is configured to release energy during thedemagnetization period.
 13. The system controller of claim 9 wherein theprotection component further includes: a timer component configured toreceive the detected voltage and generate a reference signal based on atleast information associated with the detected voltage, the referencesignal being associated with a threshold time period; and a comparatorcomponent configured to receive the reference signal and thedemagnetization signal and generate the protection signal based on atleast information associated with the reference signal and thedemagnetization signal.
 14. The system controller of claim 9 wherein theprotection component further includes: a timer-and-comparator componentconfigured to receive the detected voltage and the demagnetizationsignal and generate the protection signal based on at least informationassociated with the detected voltage and the demagnetization signal. 15.The system controller of claim 14 wherein the voltage detector includes:a sampling component configured to sample the feedback signal andgenerate a sampled signal; and a capacitor configured to receive thesampled signal and generate the detected voltage.
 16. The systemcontroller of claim 15 wherein the sampling component includes a switch.17. The system controller of claim 14 wherein the timer-and-comparatorcomponent includes: an amplifier including a first input terminal, asecond input terminal and an output terminal; a first transistorincluding a first transistor terminal, a second transistor terminal anda third transistor terminal; a second transistor including a fourthtransistor terminal, a fifth transistor terminal and a sixth transistorterminal; a third transistor including a seventh transistor terminal, aneighth transistor terminal and a ninth transistor terminal; a fourthtransistor including a tenth transistor terminal, an eleventh transistorterminal and a twelfth transistor terminal; a fifth transistor includinga thirteenth transistor terminal, a fourteenth transistor terminal and afifteenth transistor terminal; and a resistor including a first resistorterminal and a second resistor terminal; wherein: the first inputterminal is configured to receive the detected voltage; the second inputterminal is coupled to the sixth transistor terminal; the outputterminal is coupled to the fourth transistor terminal and the firstresistor terminal; the first transistor terminal is coupled to theseventh transistor terminal and the third transistor terminal; the thirdtransistor terminal is coupled to the fifth transistor terminal; theninth transistor terminal is coupled to the eleventh transistorterminal; the tenth transistor terminal is coupled to the thirteenthtransistor terminal and configured to receive a first signal related tothe demagnetization signal; the twelfth transistor terminal is coupledto the fourteenth transistor terminal and configured to output a secondsignal; and the fifteenth transistor terminal is coupled to the secondresistor terminal.
 18. The system controller of claim 17 wherein thetimer-and-comparator component further includes a comparator configuredto receive the second signal and a reference signal and generate acomparison signal related to the protection signal.
 19. The systemcontroller of claim 18 wherein the timer-and-comparator componentfurther includes a cycle-debounce component configured to receive thecomparison signal and output the protection signal.
 20. A systemcontroller for protecting a power conversion system, the systemcontroller comprising: a protection component configured to receive ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system,receive a current-sensing signal associated with a primary currentflowing through a primary winding of the power conversion system,process information associated with the demagnetization signal, thecurrent-sensing signal, and a detected voltage generated based on atleast information associated with the feedback signal, and generate aprotection signal based on at least information associated with thedetected voltage, the demagnetization signal, and the current-sensingsignal; and a driving component configured to receive the protectionsignal and output a driving signal to a switch configured to affect theprimary current flowing through the primary winding; wherein: thedetected voltage is related to an output voltage of the power conversionsystem; and the demagnetization signal is related to a demagnetizationperiod of the power conversion system; wherein the protection componentand the driving component are further configured to, if the detectedvoltage, the demagnetization signal and the current-sensing signalsatisfy one or more conditions, output the driving signal to cause theswitch to open and remain open in order to protect the power conversionsystem; wherein the protection component and the driving component arefurther configured to, in response to at least the detected voltage, thedemagnetization signal and the current-sensing signal not satisfying theone or more conditions, output the driving signal as a modulation signalto turn on and turn off the switch within a switching periodcorresponding to a modulation frequency.
 21. The system controller ofclaim 20 wherein the protection component and the driving component arefurther configured to, if the detected voltage, the demagnetizationsignal and the current-sensing signal satisfy the one or moreconditions, shut down the system controller.
 22. The system controllerof claim 20 wherein the protection component and the driving componentare further configured to, if the detected voltage, the demagnetizationsignal and the current-sensing signal satisfy the one or moreconditions, output the driving signal to open the switch without anymodulation.
 23. The system controller of claim 20 wherein the protectioncomponent and the driving component are further configured to, if thedetected voltage, the demagnetization signal and the current-sensingsignal satisfy the one or more conditions, output the driving signal toopen the switch for a period of time longer than at least the switchingperiod.
 24. The system controller of claim 20 wherein the protectioncomponent is further configured to receive the detected voltagegenerated based on at least information associated with the feedbacksignal.
 25. The system controller of claim 20 wherein the protectioncomponent is further configured to receive the feedback signal andgenerate the detected voltage based on at least information associatedwith the feedback signal.
 26. The system controller of claim 20 whereinthe detected voltage, the demagnetization signal and the current-sensingsignal satisfy the one or more conditions if the demagnetization periodis smaller in duration than a threshold time period corresponding to thedetected voltage and the current-sensing signal.
 27. The systemcontroller of claim 20 wherein the protection component includes avoltage detector configured to receive the feedback signal and generatethe detected voltage based on at least information associated with thefeedback signal.
 28. The system controller of claim 27 wherein thevoltage detector is configured to receive the feedback signal from avoltage-signal generator coupled to an auxiliary winding of the powerconversion system.
 29. The system controller of claim 28 wherein thevoltage-signal generator is configured to generate the feedback signalbased on at least information associated with the output voltage, theoutput voltage being related to a secondary winding of the powerconversion system.
 30. The system controller of claim 29 wherein thesecondary winding is configured to release energy during thedemagnetization period.
 31. The system controller of claim 27 whereinthe protection component further includes: a peak-detection componentconfigured to receive the current-sensing signal and generate athreshold signal based on at least information associated with thecurrent-sensing signal; a timer component configured to receive thedetected voltage and the threshold signal and generate a referencesignal based on at least information associated with the detectedvoltage and the threshold signal, the reference signal being associatedwith a threshold time period; and a comparator component configured toreceive the reference signal and the demagnetization signal and generatethe protection signal based on at least information associated with thereference signal and the demagnetization signal.
 32. The systemcontroller of claim 27 wherein the protection component furtherincludes: a peak-detection component configured to receive thecurrent-sensing signal and generate a threshold signal based on at leastinformation associated with the current-sensing signal; and atimer-and-comparator component configured to receive the detectedvoltage, the demagnetization signal and the threshold signal andgenerate the protection signal based on at least information associatedwith the detected signal, the demagnetization signal and the thresholdsignal.
 33. The system controller of claim 32 wherein the voltagedetector includes: a sampling component configured to sample thefeedback signal and generate a sampled signal; and a capacitorconfigured to receive the sampled signal and generate the detectedvoltage.
 34. The system controller of claim 33 wherein the samplingcomponent includes a switch.
 35. The system controller of claim 32wherein the timer-and-comparator component includes: an amplifierincluding a first input terminal, a second input terminal and an outputterminal; a first transistor including a first transistor terminal, asecond transistor terminal and a third transistor terminal; a secondtransistor including a fourth transistor terminal, a fifth transistorterminal and a sixth transistor terminal; a third transistor including aseventh transistor terminal, an eighth transistor terminal and a ninthtransistor terminal; a fourth transistor including a tenth transistorterminal, an eleventh transistor terminal and a twelfth transistorterminal; a fifth transistor including a thirteenth transistor terminal,a fourteenth transistor terminal and a fifteenth transistor terminal;and a resistor including a first resistor terminal and a second resistorterminal; wherein: the first input terminal is configured to receive thedetected voltage; the second input terminal is coupled to the sixthtransistor terminal; the output terminal is coupled to the fourthtransistor terminal and the first resistor terminal; the firsttransistor terminal is coupled to the seventh transistor terminal andthe third transistor terminal; the third transistor terminal is coupledto the fifth transistor terminal; the ninth transistor terminal iscoupled to the eleventh transistor terminal; the tenth transistorterminal is coupled to the thirteenth transistor terminal and configuredto receive a first signal related to the demagnetization signal; thetwelfth transistor terminal is coupled to the fourteenth transistorterminal and configured to output a second signal; and the fifteenthtransistor terminal is coupled to the second resistor terminal.
 36. Thesystem controller of claim 35 wherein the timer-and-comparator componentfurther includes a comparator configured to receive the second signaland the threshold signal and generate a comparison signal related to theprotection signal.
 37. The system controller of claim 36 wherein thetimer-and-comparator component further includes a cycle-debouncecomponent configured to receive the comparison signal and output theprotection signal.
 38. A method for protecting a power conversionsystem, the method comprising: receiving a demagnetization signalgenerated based on at least information associated with a feedbacksignal of the power conversion system; processing information associatedwith the demagnetization signal and a detected voltage generated basedon at least information associated with the feedback signal; generatinga protection signal based on at least information associated with thedetected voltage and the demagnetization signal; receiving theprotection signal; generating a driving signal based on at leastinformation associated with the protection signal; and outputting thedriving signal to a switch configured to affect a primary currentflowing through a primary winding of the power conversion system;wherein: the detected voltage is related to an output voltage of thepower conversion system; and the demagnetization signal is related to ademagnetization period of the power conversion system; wherein theoutputting the driving signal to a switch configured to affect a primarycurrent flowing through a primary winding of the power conversion systemincludes, if the detected voltage and the demagnetization signal satisfyone or more conditions, outputting the driving signal to cause theswitch to open and remain open in order to protect the power conversionsystem; wherein the outputting the driving signal to a switch configuredto affect a primary current flowing through a primary winding of thepower conversion system further includes, in response to at least thedetected voltage and the demagnetization signal not satisfying the oneor more conditions, outputting the driving signal as a modulation signalto turn on and turn off the switch within a switching periodcorresponding to a modulation frequency.
 39. A method for protecting apower conversion system, the method comprising: receiving ademagnetization signal generated based on at least informationassociated with a feedback signal of the power conversion system;receiving a current-sensing signal associated with a primary currentflowing through a primary winding of the power conversion system;processing information associated with the demagnetization signal, thecurrent-sensing signal, and a detected voltage generated based on atleast information associated with the feedback signal; generating aprotection signal based on at least information associated with thedetected voltage, the demagnetization signal, and the current-sensingsignal; receiving the protection signal; generating a driving signalbased on at least information associated with the protection signal; andoutputting the driving signal to a switch configured to affect theprimary current flowing through the primary winding; wherein: thedetected voltage is related to an output voltage of the power conversionsystem; and the demagnetization signal is related to a demagnetizationperiod of the power conversion system; wherein the outputting a drivingsignal to a switch configured to affect the primary current flowingthrough the primary winding includes, if the detected voltage, thedemagnetization signal and the current-sensing signal satisfy one ormore conditions, outputting the driving signal to cause the switch toopen and remain open in order to protect the power conversion system;wherein the outputting the driving signal to a switch configured toaffect a primary current flowing through a primary winding of the powerconversion system further includes, in response to at least the detectedvoltage, the demagnetization signal and the current-sensing signal notsatisfying the one or more conditions, outputting the driving signal asa modulation signal to turn on and turn off the switch within aswitching period corresponding to a modulation frequency.